1. Field of the invention
This invention relates to a recording medium, and more particularly to a recording medium for writing and/or reading information by means of laser beam of high energy density.
2. Description of prior Arts
Information recording media utilizing a beam of high energy density such as laser beam have been developed in recent years and have been put to practical use. Such recording medium is generally called an "optical disc", and the practical applications thereof have been found, for example, as video disc and audio disc as well as disc memory for large-capacity computer and large-capacity static image file.
Writing of information on the optical disc can be conducted, for example, by irradiating the optical disc with a laser beam. Under irradiation with the laser beam, the irradiated area of the recording layer of the optical disc absorbs the energy of beam and a rise in temperature locally occurs and as a result, the recording layer is removed through evaporation or fusion on the area having received the irradiation, whereby the recording of information can be made. Reading of information from the optical disc is also conducted by irradiating the optical disc with laser beam. The information can be reproduced by detecting reflected light or transmitted light corresponding to the alteration in the optical characteristics of the recording layer.
The optical disc generally comprises a disc-shaped transparent substrate of a plastic or glass material and a recording layer composed of a metal or a semi-metal such as Bi, Sn, In or Te which is provided on the substrate. It has been reported in Japanese Patent Publications No. 58(1983)-33120 and No. 58(1983)-15319 that the sensitivity of the recording layer containing the metal or semi-metal can be enhanced by incorporation of a metal sulfide such as GeS, a metal fluoride such as MgF.sub.2 or a metal oxide such as MoO.sub.2 into the layer.
Further, a subbing layer of a ploymer material may be provided between the substrate and the recording layer so that the adhesion between the substrate and recording layer is ensured and the sensitivity of the optical disc is enhanced.
Furthermore, Japanese Patent Provisional Publication No. 50(1985)-181151 proposes to provide a metal film between the substrate and recording layer for enhancing the sensitivity of the recording layer.